The invention relates to workpiece machining systems and, more particularly, to a system and method for clamping of a pallet carrying a workpiece to a support stand for aligning the workpiece and giving it the necessary stiffness for accurate machining thereof.
For machining of workpieces, special requirements or unique workpieces often govern the selection of a specific type of machine and fixturing. For instance, the size and configuration of the workpiece may affect the stiffness or rigidity with which a part can be held while being machined. In machining of large parts, a major consideration is the rigidity of the fixturing of the large part. An effective fixture for machining some large parts such as aircraft parts is a problem because of their great size in relation to wall thickness. Nevertheless, rigidity of the machine and setup is essential for optimum results. Lack of sufficient rigidity in the tooling and workpiece setup can result in tool deflection and chatter causing excessive tool wear and breakage, damage to workpieces, dimensional inaccuracy, and unacceptable surface finish.
Large and/or extremely heavy parts are usually cut by machines designed specifically for these parts. The present invention will be described in connection with one use thereof which is the profile machining of large aircraft parts; but the invention is not limited to this particular use of the invention. Large aircraft parts can be machined by profilers which are high and very long machines having horizontal spindles for advancing the cutting tool transversely relative to high and long vertical surfaces on the large parts for machining thereof. Because these large, heavy parts are not lying flat and are upstanding when being machined, there generally is greater instability introduced into the fixturing, making it very difficult to mount the part so that it is held with sufficient rigidity and has the necessary stiffness to provide for accurate machining results.
The profilers are machine tools that can generally include support structure and carriages, e.g., columns and rams, driven as by linear motors for carrying spindles along three axes of movement for high speed and high force machining along the entire extent of the large aircraft component and which can also include a nutating head for machining at various angles into the workpiece transversely to the vertical. These types of machines represent a major capital investment so that their use is generally restricted to removing large amounts of metal from massive workpieces like aircraft components. In machining large aircraft components, skin milling, which involves deep cuts, i.e., on the order half an inch or more, is commonplace. In rugged machining operations such as these where there are deep, heavy cuts, the workpiece will commonly be subjected to such great force, e.g. on the order of approximately 500 lbs. of cutting force, that it will bend and shift if not firmly secured which, as previously discussed, can be especially difficult with large, heavy workpieces that are arranged vertically. In this regard, stiffness of the part being machined is critical so as to avoid damage to the workpiece by miscuts, and damage to the tools and machine.
Accordingly, with the above-described profiler machines, the aircraft parts are mounted and clamped to a fixture plate that has, in turn, been clamped to a large thick and stiff pallet which gives the part the stiffness or rigidity necessary for accurate machining. In other words, it is the large thick pallet that provides the heavy fixture to add sufficient rigidity to the setup for the large part. Once the part is properly mounted to the pallet, it is then transported to the profiler machine for machining. This rigid setup minimizes or eliminates the previously discussed adverse conditions that can occur when machining large, relatively thin cross-section aircraft components in terms of obtaining proper fixturing, thereby increasing production and lowering production cost, and also allowing for increased feed rates and spindle speeds to be utilized which further increases production rates.
A shortcoming of the use of the pallet to provide the part the required stiffness is that the pallet itself has to be sufficiently massive for this purpose. Before setup, the pallets have to be transported from storage to the setup location. Once the part is mounted to the pallet, the massive pallet and attached part have to be moved to the profiler machine for machining. Where the pallet""storage is removed from where the part setup is to occur, and also where the setup of the part to the pallet is at some distance from the machine tool, the sheer size of the pallet and attached part can create problems in terms of time, equipment and manpower in transporting this assembly to the machine. Thus, there is a need for a machining system for workpieces, such as large, heavy thin-walled aircraft components, which provides sufficient stiffness to the part while being machined without the drawbacks afforded by the use of large, thick and heavy pallet fixtures.
In accordance with the present invention, a new and improved machining system is provided for machining large, heavy workpieces that are oriented vertically, such as large aircraft components. The machining system includes a workpiece support and a transportable thin pallet for being selectively magnetically clamped to the support. The pallet is large enough to support the large workpiece but is still relatively thin and light so that it can be easily transported to the machining station where the workpiece support is located. Due to the very large pallet surface area for mounting of the large workpiece, and because it is not very thick, e.g., approximately 8 inches, and has an open frame construction, the pallet on its own cannot impart the stiffness to the part necessary for accurate machining. The much thicker upstanding support is utilized to provide the pallet and affixed part the necessary stiffness when the pallet is clamped thereto, by a magnetic clamping arrangement that will be described hereinafter. In this manner, it is the larger workpiece support with its greater mass and stiffness vis-a-vis the transportable pallet that provides the workpiece mounted to the pallet the necessary stiffness so that it can be accurately machined. And because the pallet is relatively thin and flexible, it can more easily be maneuvered and transported than if it were the pallet that had to provide the stiffness for machining which would require it to be much more massive akin to the pallets previously utilized and thus making it much more difficult to transport. Thus, the present invention keeps workpieces held in a fixed and stationary position when they are being machined but not when they are being transported so that the weight of a large, heavy and massive pallet need not be transported.
In accordance with an important aspect of the invention, the use of magnetic attraction forces to clamp the pallet to the support results in quick assembly and disassembly of the pallets to the support. This reduces machining cycle time and increases production rates. Additionally, the magnetic forces provide a strong uniform clamping force across the entire area of the pallet that makes it strong and rigid like the stand support during the machining operation.
In one form of the invention, a machining system is provided for machining heavy workpieces by a cutting machine located at a predetermined machining position. A workpiece support upstanding at the machining position receives and holds a pallet in a vertical position during a machining operation on a vertical surface of the workpiece. A transportable pallet is provided having a vertical side and a workpiece affixed to the vertical side at a precise position thereon for being machined by the cutting machine while the pallet is on edge and being vertically positioned. Electromagnets and magnetizable areas are provided on the work support and the transportable pallet to magnetically clamp the transportable pallet to the workpiece support. A transporter transports the transportable pallet with the workpiece to and from the workpiece support. Registering members on the workpiece support and on the transportable pallet are brought into engagement to register the pallet on the workpiece support to locate the workpiece on the pallet at a predetermined position on a vertical side of the workpiece support. The upstanding workpiece support has a mass and stiffness substantially greater than the mass and stiffness of the transportable pallet. The transportable pallet has a stiffness substantially less than needed for the machining operation by the cutting machine and the magnet clamping of the electromagnets and magnetizable areas causes the pallet and upstanding workpiece support to provide a combined stiffness for the workpiece projecting from the vertical, on-edge pallet to permit accurate machining of the workpiece. The present machining system provides good fixturing for large workpieces that are machined in a vertical orientation and are subjected to large cutting forces transversely directed relative to the vertical by the above-described magnetic clamping arrangement while also allowing the pallet to which the workpieces are to be mounted to be transported in a relatively easy and efficient manner.
The upstanding workpieces support can be several times thicker in the horizontal direction than the thickness of the transportable pallet. As stated, the large, thin transportable pallet is relatively flexible in comparison to the upstanding support with the thicker support imparting the strength and rigidity or stiffness necessary for accurate machining, and the flexible pallet being of less mass so that it is adapted for being transported to and from the machining location.
Preferably, a set-up stand is also provided with electromagnets and magnetizable areas on the set-up stand. The transportable pallet is magnetically clamped to the set-up stand to allow a workpiece to be positioned on and secured to the transportable pallet. A pallet storage station can be provided to hold and store a plurality of pallets with the transporter carrying pallets from the pallet storage station to the set-up stand and from the set-up stand to the machining position.
In one form, the transporter can include a lift mechanism to lift the transportable pallet and to lower the pallet onto stops on the workpiece support. A pull-in device pulls the pallet laterally to bring the electromagnets and magnetizable areas into close proximity. In this manner, when the magnets are energized, the impact between the electromagnets and magnetizable areas is minimized so as to reduce the amount of rubbing and wear therebetween.
In another form of the invention, a pallet holding apparatus is provided including a large fixed stationary stand for imparting stiffness to a relatively thin and flexible pallet to be held thereto. The stand has a vertical wall to which the pallet is held for either positioning and fixing a workpiece to the pallet or machining a fixed workpiece carried by the pallet. A plurality of magnets are spaced across the wall for exerting a magnetic force to bring the pallet toward the magnets for securing the pallet in a vertical orientation to the vertical wall of the stand without requiring mechanical or hydraulic type clamping mechanisms and services therefor for rapid attachment and detachment of the pallet to and from the stand. A plurality of corresponding magnet engaging areas are associated with the pallet with the areas being of a material that is attracted by the magnetic force so that the areas register against the magnets when the magnets are activated. The magnets and areas are arranged to spread out and balance the magnetic force to evenly draw the pallet against the stand wall across the entire extent of the pallet. A controller activates the magnets in a predetermined sequence for bringing the magnets and pallet areas into engagement with each other. The use of magnetic clamping of the pallet to the upstanding support obviates the need for mechanical and hydraulic clamping devices which generally require greater set-up time, and the associated lube and hydraulic oil service lines therefor. In this manner, the present system simplifies the equipment required for clamping the transportable pallet to the support, reducing maintenance costs and the probability of down time.
The pallet magnet engaging areas can be formed on pads attached to the pallet with the magnets and pads being arranged in horizontal rows across the stand and pallet. The controller activates the pallets in a bottom to top sequence with the lower rows of magnets activated for drawing in corresponding lower rows of pallet pads thereto before higher rows of magnets are activated.
In one form, the magnets are electropermanent magnets and the controller includes circuitry connected to the magnets with the controller activating or deactivating the magnets by supplying an electrical charge thereto via the circuitry without requiring continuing power to the magnets for magnetically attracting and holding the corresponding pallet magnet engaging areas thereto or releasing the areas therefrom. In this manner, power interruptions do not affect the magnetic holding force applied by the magnets to the pallet magnet engaging areas.
In one form, the pallet is approximately 10 to 20 meters in length. As previously stated, the stand can be several times thicker than the pallet, and preferably it is approximately seven to eight times thicker than the pallet.
In one form, each of the magnets exert between approximately 800 pounds to approximately 1200 pounds of magnetic force.
In a preferred form, the stand includes a shiftable way mechanism and has horizontal rests, and the pallet includes bearings and vertical locating members. The way mechanism has a support position with the pallet bearings being supported by the stand way mechanism, and a lowered position with the way mechanism shifted so that the pallet locating members engage the rests to locate the pallet vertically relative to the stand and with the weight of the pallet supported by the rests. The rests and locating members are disposed toward the bottoms of the stand and pallet, respectively, and the controller preferably activates the magnets in a bottom to top sequence for sliding the locating members along the stand rests to minimize and overcome the friction therebetween as the pallet is drawn toward the stand, and specifically against the magnets thereon. In a preferred form, the magnets disposed toward the bottom of the stand exert a larger magnetic force than the magnets thereabove for pulling the locating members with a greater force along the rests than the higher magnets.
In one form, the longitudinal and vertical alignment mechanisms cooperate between the stand and pallet for substantially aligning the magnets with the corresponding pallet areas prior to activation of the magnets by the controller.
Spacer members can be mounted between each magnet and the stand wall for being released to allow the magnets to be pulled off from the corresponding pallet areas for detaching the pallet from being magnetically held to the wall when power to the controller fails. The spacer members may include bolts for attaching the magnets thereto, with removal of the bolts releasing the spacer members from behind the magnets to allow the bolts to be reinserted for connecting to the magnets magnetically held on the corresponding pallet areas for pulling the magnets with the bolts off therefrom.
Another aspect of the present invention is a method of imparting strength and stiffness to a thin, flexible pallet on which a part to be machined is fixed. The method includes transporting the pallet from a first location to a second location, moving the pallet so that one side thereof is vertically oriented and faces a vertical wall of a large thick stationary stand at the second location, aligning magnets and corresponding magnet engaging areas on the stand and pallet with each other, activating the magnets to draw the pallet securely against the stand to provide stiffness and rigidity to a part during machining despite the flexibility of the thin pallet, and arranging the magnets so that the magnetic force exerted between the stand and pallet is spread out and balanced across the side of the pallet facing the stand for evenly drawing the pallet against the stand.